Described herein are metallization contact structures and methods for forming multiple-layer electrode structures for silicon photovoltaic cells (hereinafter “silicon solar cells”). The metallization contact structures and methods for forming multiple-layer electrode structures provide silicon solar cells having low contact resistance with a small contact area and lowered surface recombination. The silicon solar cells provided by the metallization contact structures and methods maintain high conductivity, solderability and stability of current via a silver gridline electrode.
Solar cells are typically photovoltaic devices that convert sunlight directly into electricity. Solar cells typically include a silicon semiconductor that absorbs light irradiation, such as sunlight in a way that creates free electrons, which in turn are caused to flow in the presence of a built-in field to create direct current (hereinafter “DC”) power. The DC power generated by several PV cells may be collected on a grid placed on the cell. Current from multiple PV cells is then combined by series and parallel combinations into higher currents and voltages. The DC power thus collected may then be sent over wires, often many dozens or even hundreds of wires.
The state of the art for metallizing silicon solar cells for terrestrial deployment is screen printing. Screen printing has been used for decades, is a robust, simple, rapid, and cost-effective metallization method and can be easily automated for large-scale solar cell manufacturing. In a conventional screen printing approach to metallization solar cells, a squeegee presses a paste through a mesh with an emulsion pattern that is held over the wafer. A typical paste for solar cell metallization consists of a mixture of silver particles and a glass flit in an organic vehicle. When the wafer is fired, the organic vehicle decomposes and the glass flit softens and then dissolves the surface passivation layer and creating a pathway for the silver to reach silicon by forming a multitude of random points under a silver pattern formed by the paste. The surface passivation, which may also serve as an anti-reflection coating, is a dielectric layer, such as a silicon nitride layer and is an essential part of the cell covering the cell except for electrical contact areas. Upper portions of the paste densify into one or more metal thick films that carries current from the cell. These films form gridlines on a front-side of the wafer, and a base contact on a backside of the wafer. The silver of the paste is also a surface to which tabs connecting adjacent cells may be soldered.
While the glass frit approach combined with screen printing to opening contacts has an advantage, such that no separate process step is needed to open the passivation, the glass frit approach has significant drawbacks which limit the further improvement of cell efficiency. First, contact resistance is very large, for example, specific contact resistance between the semiconductor emitter layer (sun-exposed surface) and the silver gridline is at the order of about 10−3 Ω·cm2. This specific contact resistance between the semiconductor emitter layer and the silver gridline is several orders of magnitude higher than the specific contact resistance that may be reached in semiconductor integrated circuit devices, which is at the order of about 10−7 Ω·cm2. Due to this large specific contact resistance, the emitter layer in a solar cell must be heavily doped and large contact area between the emitter and silver gridline must be used, otherwise the silver of the paste cannot make good electrical contact to the silicon. The heavy doping kills the minority carrier lifetime in the top portion of the cell and limits the blue response of the cell, and the large contact area generates higher surface recombination rate. As a result, the overall efficiency of the solar cell is reduced. Another problem with the glass flit approach is a narrow process window. The narrow process window may be a problem because a thermal cycle, that fires the gridline, must burn through the silicon nitride to provide electrical contact between the silicon and the silver without allowing the silver to shunt or otherwise damage the junction. This narrow process window severely limits the process time to the order of about 30 seconds and temperature band to about 10° C. around the peak firing temperature.
Ideally, a metallization technology for silicon solar cells should form the gridline electrodes with low specific contact resistance and thus low contact area, high conductivity, good solderability, and long time stability. Because it is very difficult for a single layer electrode to meet all these requirements, several methods for forming multiple-layer electrode strictures have been proposed for silicon solar cells.
U.S. Patent Publication No. 2007/0169806 A1 (filed on Jan. 20, 2006) discloses forming multiple-layer gridline front surface electrodes by forming contact holes through the passivation layer using a non-contact patterning apparatus such as a laser-based patterning system. The contact holes may be filled by inkjet printed nanophase metallic inks and covered with silver gridlines. However, several problems associate with using printed nanophase metallic inks for filling the contact holes include quality and availability of the nanophase metallic inks, the wetting behavior and contact characteristic between the nanophase metallic ink and the silicon surface in the contact holes, and the process compatibility of nanophase metallic ink with firing silver gridlines.
U.S. Patent Publication No. 2004/0200520 A1 (filed on Apr. 10, 2003) discloses a multiple-layer backside electrode structure that is formed by making contact holes through chemically etching the passivation or anti-reflection coating layer, followed by sputtering or evaporating a three layer-seed metal stack to form the contact with emitter and plating copper and a thin metal capping layer to form gridlines. However, chemically etching the passivation layer involves several extra process steps including applying an etch resist layer, patterning the etching resist layer, and striping off the etching resist layer after patterning the passivation layer.
U.S. Patent Publication No. 2005/0022862 A1 (filed on Aug. 1, 2003) discloses screen printing a liquid ink pattern layer devoid of particles onto the silicon oxide passivation layer to form a particle-devoid ink pattern layer as an etching protection mask. However, the particle-devoid ink pattern layer must be stripped off after etching the silicon oxide layer.
It is therefore deemed desirable to develop cost efficient, simple and non-complex metallization contact structures and methods for forming multiple-layer electrode structures for silicon solar cells that provide low contact resistance, low contact area, high conductivity, high solderability, and high stability from solar exposure. Such metallization contact structures and methods for forming multiple-layer electrode structures may overcome problems associated with current single layer electrode structures as well as the complexity and uncertainty associated with known prior art approaches for forming multiple-layer electrode structures.